1. Field of the Invention
The present invention relates to a receiver in a digital communication system utilizing an Orthogonal Frequency Division Multiplexing (OFDM) method, and more particularly to a method for performing frame synchronization by using characteristics of synchronization word inverted at each frame in a transmission parameter signaling (TPS) block, and an apparatus employing the same.
2. Description of the Prior Art
In a wireless communication channel and digital high-definition TV (HDTV) transmission channel, it is known that an Inter-Symbol Interference (ISI) caused by multi-path fading in a received signal commonly occurs. Particularly, when data for HDTV are transmitted through the channel at high speed, the ISI increases causing errors to be generated during the data recovery at the receiving side. To solve this problem, recently, OFDM method has been proposed as a transmission method for use in the Digital Audio Broadcasting (DAB) and Digital Terrestrial Television Broadcasting (DTTB) standards.
In OFDM method, serially-inputted symbol streams are divided into a predetermined unit block. The divided symbol streams of each unit block are converted into N number of parallel symbols. The N number of parallel symbols are multiplexed and added by using a plurality of subcarriers having different frequencies, respectively, according to Inverse Fast Fourier Transform (IFFT) algorithm. The added data are transmitted via the channel. That is, the N number of parallel symbols are defined as one unit block, and each subcarrier of the unit block has an orthogonal characteristic, which does not have an influence on subchannels. Compared to a conventional single carrier transmission method, OFDM method can reduce the ISI caused by the multi-path fading by maintaining the same symbol transmission rate and increasing symbol period as much as by the number of subchannels (N). Especially, in OFDM method, a guard interval (GI) is inserted between the transmitted symbols to enhance the capability of the ISI reduction, making it possible to realize a simplified structure of channel equalizer. In contrast to a conventional Frequency Division Multiplexing (FDM) type, OFDM method has a characteristic that spectrums of each subchannel are superimposed causing it to have a higher band efficiency. Further, the spectrum has a wave of rectangular shape and electric power is uniformly distributed at each frequency band, which prevents from being affected by the same channel interference. The OFDM method is commonly combined with modulation types such as Pulse Amplitude Modulation (PAM), Frequency Shift Keying (FSK), Phase Shift Keying (PSK), and Quadrature Amplitude Modulation (QAM).
FIGS. 1A to 1B are format diagrams of transmission symbol units of a conventional OFDM signal. Symbols transmitted from a transmitting side, as shown in FIG. 1A, comprises an useful part and a guard interval. The useful part contains useful OFDM samples, and the guard interval is inserted in front side of the useful part and separates OFDM samples into symbol units. Samples used in the guard interval are copies of samples located in lower portion of the useful part. According to DTTB standard, the size of the useful part is separated into 2K mode and 8K mode by a Fast Fourier Transform (FFT) size. For 2K mode, as shown in FIG. 1B, the size of the useful part is defined by "2048" samples. In addition, the size of the guard interval is separated into 1/4, 1/8, 1/16, and 1/32 of the FFT size. In case of 1/4 of the FFT size, as shown in FIG. 1B, the size of the guard interval is defined by "512" samples. Here, "2048" is the sum of 1705 useful subcarriers and 343 NULL subcarriers. The guard interval is comprised of copied data from the last parts of the useful part, 1536-th data to 2047-th data (namely, 512 sizes). The guard interval is inserted in the front portion of the useful data. Finally, the size of transmission symbol units is defined by the sum (2560) of the useful part (2048) and the guard interval (512).
Meanwhile, according to DVB standard, an OFDM signal comprises frames having has 68 OFDM symbols, respectively and a super frame comprises four frames. Each frame comprises transmitted data, Continual Pilot Carriers (CPC), and a TPS pilot.
The transmitting side of the OFDM communication system performs IFFT for N number of symbols, defined as one block unit, and transmits it in frame units. The receiving side performs the FFT for the transmitted frame, to recover an original information. Accordingly, when the frames between the transmitting and receiving sides are not synchronized, errors are generated during the recovery of data.